MRI is a well-established imaging technique. A conventional MRI device can provide a homogenous magnetic field (conventionally referred to as B0), which may be aligned along an axis in a human body to be imaged. The magnetic field can cause nuclear spins to align, and their magnetic moments may then precess around the direction of the main magnetic field. A subsequent radiofrequency pulse (e.g., an excitation pulse) can be applied by a radio frequency (“RF”) transmit coil, thereby causing the nuclear spins to absorb energy. This can perturb a magnetic moment of the nuclear spins around the axis of B0.
After the excitation pulse, the precessing magnetic moments can return to a steady state by a process referred to as free induction decay (“FID”). During FID, the absorbed energy may be released, and the NMR signals can be detected by a RF coil, which may be configured to only transmit or to both transmit and receive. The measured signal can be processed to extract additional information such as, e.g., data that may be used to generate cross-sectional images of the human body, or generation and examination of spectroscopic data. Pulse sequences can be developed and/or refined to improve the quality and type of image data. More specifically, radiofrequency pulses and field gradients can be varied to create images and/or data suitable for various clinical and research purposes.
When using MRI techniques, it can be desirable to have excitation and/or reception interactions be spatially uniform in the imaging volume. This can, e.g., provide better image uniformity. Excitation field homogeneity can be obtained by using a whole-body volume RF coil for transmission. An NMR signal generated by excited tissue can then be received either by the body coil itself, or by another coil such as, e.g., a surface coil which can be located immediately adjacent to the NMR signal region of interest. The surface coil can be configured as a “transmit only” coil or a “transmit/receive” coil. A whole body volume coil that is utilized as a receive coil may produce a lower signal-to-noise ratio (“SNR”) than a local surface coil, because it may be located farther from the signal-generating target. Surface coils can also reduce noise contributions from electrical losses in the object being imaged, which can result in a higher SNR as compared to a remote coil. This can result in an improved image quality or data acquisition.
A high SNR can be achieved for a region being imaged by using a surface coil element having a diameter, D, which is approximately equal to the distance from the coil to the region being imaged. The region of tissue being imaged can often be located directly below a surface coil at approximate depth (“DP”) from the coil itself. As the diameter of a surface coil element is decreased, the corresponding SNR generated using the coil element may increase, which can provide better image or data quality. A field-of-view (“FOV”) that can be obtained using the surfaced coil may decrease with a decreasing coil element size. The size of a surface coil can thus be selected to provide a compromise between improved resolution and a larger field-of-view.
Two or more coil elements may be combined in a surface coil to increase the effective FOV. A surface coil having more than one conductive element and that is configured to minimize or reduce mutual inductance between the elements may be referred to as a phased array coil or a multi-channel phased array coil. A phased array coil can maintain a high SNR associated with individual coil elements, while enlarging the overall FOV and minimizing or reducing noise and/or artifacts in the image data obtained. However, a mutual inductance between the coil elements can cause a decrease in the frequencies detected and/or resolved, and may degrade the SNR associated with the involved loops.
A phased array arrangement configured to combine signals from an array of smaller coils to obtain a higher SNR than may be achieved using a single large diameter loop coil with the same FOV is described, e.g., in U.S. Pat. No. 4,825,162 to Roemer et al. (“Roemer”). Roemer describes certain techniques for decoupling both adjacent and non-adjacent coil elements. The adjacent coil elements can partially overlap in certain arrangements to eliminate a mutual impedance within these coil elements (e.g., “next-to interactions”). Mutual impedance in non-adjacent elements (e.g., “next-to-nearest interactions”) within a plurality of coil elements can be reduced or eliminated by, e.g., connecting individual elements to preamplifier units which may have a low impedance, but which may have a high impedance with respect to individual elements. This decoupling can reduce or prevent current flow in an antenna in response to an induced voltage. The magnetic field associated with such a current could induce signals in a neighboring antenna in the absence of such decoupling.